1. Field of the Invention
This invention relates to a perpendicular magnetic recording floppy disk.
2. Description of the Prior Art
A perpendicular magnetic recording system is now being developed, in which magnetization perpendicular to the surface of a magnetic recording medium is utilized to attain high-density recording. In the perpendicular magnetic recording system, an increase in recording density permits a decrease in the demagnetizing field, making it easy to increase the recording density. The magnetic recording medium of this type is formed by sputtering a Co-Cr-based magnetic material on a substrate such that the axis of easy magnetization is in a direction perpendicular to a recording surface. A magnetic tape and floppy disk are known which are manufactured by forming a Co-Cr-based thin layer on a plastic film substrate made of, for example, polyimide or polyester.
When the Co-Cr-based thin layer is formed, by sputtering, on one surface of the plastic film substrate of this type, a very large curl occurs in the thin film substrate with the thin-layer-deposited side of the film substrate curled to the outside. The extent of the curl depends upon the substrate temperature during deposition. That is, if the substrate temperature is lowered, the curl of the substrate can be suppressed to a corresponding extent. A perpendicular coercive force Hcv also has a close relation to the substrate temperature. That is, if a Co-Cr-based thin layer is formed at a lower substrate temperature, the coercive force Hcv is markedly lowered. From this it can be appreciated that there exists a trade-off between the curl of the film substrate and the coercive force Hcv with respect to the substrate temperature. In order to solve such a trade-off, a Co-Cr-based thin layer with the same thickness is formed on both the side of the plastic film substrate, so that a curl produced on one side of the film substrate is cancelled to a certain extent by a curl produced on the other side of the film substrate. The inventors have found by experiments that the curl of the substrate cannot be effectively cancelled by merely forming a Co-Cr-based thin layer with the same thickness on both the sides of the substrate. That is, the mere formation of such a Co-Cr-based thin layer on both the sides of the substrate could not provide a complete flatness to the plastic film substrate. Unless a high degree of flatness is attained, a very great problem is presented in the high-density signal recording. A slight curl of the substrate causes a decrease in the reproduction output and a degradation in the envelope of an output signal, thus preventing high-density signal recording. In the manufacture of floppy disks, the greatest task is to eliminate the curl of the substrate.
A method for minimizing the curl in a Co-Ni-alloy-evaporated thin film tape is disclosed in T. Fujita et al. "Curling of Metal Evaporated Thin Film Tape", Digests of the 5th Annual Conference on Magnetics in Japan, 1981, p. 77. According to this method, Al or Co-Ni alloy is back-coated on the thin film tape. This method, however, is different from the method of this invention in process and effects.